Hitherto, devices that encrypt digital content and transmit the encrypted content have been available. For example, such a device in related art compresses digital content. The device then encrypts the compressed digital content by using a single encryption key or the like. The device then transmits the encrypted digital content to a user terminal, such as a PC (personal computer), a mobile phone, or the like. Examples of the digital content include a moving image, music, and a book. The reason why the digital content is encrypted is, for example, to protect copyrights.
Upon receiving the digital content, the user terminal decrypts the digital content. The user terminal then decompresses the decrypted digital content so as to enable use of the digital content.
However, for example, information of the encryption key, used for the encryption could be guessed. More specifically, in the related art, for example, since digital content is typically encrypted with a single encryption key, the encryption key is more likely to be deciphered from the overall encrypted digital content than a case in which the digital content is encrypted with multiple encryption keys. There is also a scheme in which digital content is divided into multiple pieces of data, each having a few kilobytes to tens of kilobytes, and the divided pieces of data are encrypted with respective different encryption keys. Such a scheme, however, involves management of the encryption keys for the respective divided pieces of data.